Many refrigeration, heating and air-cooling and air-heating systems operate using a vapor compression cycle. This cycle consists of compressing refrigerant gas from a low pressure to a high pressure. The high-pressure gas exits a device used for increasing the pressure of the refrigerant, typically a compressor, and is condensed to a liquid, typically by a heat exchanger called a condenser, where heat is removed from the refrigerant. The high-pressure liquid refrigerant is lowered in pressure through an expansion device (valve or capillary tube) and the refrigerant liquid expands to a gas through a second heat exchanger called an evaporator where heat is absorbed by the refrigerant. This process of removing and absorbing heat produces a heating and cooling affect, which can be used for refrigeration, air-conditioning, and heating.
While all of these steps are important, the compression step is a crucial part of the cycle (Fluorocarbon Refrigerants Handbook, R. C. Downing, Prentice-Hall, Inc. 1988). When a compressor is used to mechanically increase the pressure of a refrigerant, a lubricant (i.e. oil) is needed in the compressor to lubricate the compressor bearings and other moving parts, and the properties of the oil must be suitable for this purpose. Often the oil leaves the compressor by slipping (i.e. leaking) past the piston rings in reciprocating compressors, by entrainment with the refrigerant, and by excessive foaming as refrigerant is released from solution in the oil. A small amount of oil circulating with the refrigerant may not cause any significant problems; however, if the oil accumulates in the system over time, the oil level in the compressor can become critically low and the compressor bearings and other moving parts can overheat and fail. Many sealed (hermetic) compressors are used for refrigerators, window air-conditioners, residential heat pumps, and commercial air-handlers. These compressors are precharged at the factory with the correct amount of oil, and a drop in oil level can reduce the life expectancy for these machines.
Problems related to compressor oil migration in a vapor compression system are related to the fact that oil may accumulate in the evaporator and cause a decrease in the cooling capacity of the system. Designers of refrigeration and air-conditioning systems often install oil separators in the discharge line of the compressor to trap oil and return it to the compressor crankcase. The oil is pushed along in the condenser by the liquid refrigerant and in the evaporator and suction line back to the compressor by the velocity of the refrigerant gas. The refrigerant pipes can also be designed to allow the oil to flow downhill back to the compressor using gravity. When oil leaves the compressor, however, it may or may not be soluble in the refrigerant, and the more soluble the refrigerant is in the oil, the better oil return to the compressor will be. The refrigerant dissolves in the oil, and reduces the viscosity of the oil that assists with the oil moving through the piping back to the compressor. Although these measures can be helpful, they are not 100% effective and may postpone the problem rather than solve it completely. For example, when there is a drop in the cooling capacity of the system, high temperatures in the compressor can lead to thermal breakdown and copper plating on valve surfaces, thus interfering with the proper operation of the compressor. Sludging and acid formation may limit the life of the system.
Several classes of commercial oils exist. Historically, the most common lubricants were natural or mineral-based oils (MO). Initially, when most refrigerants were based on chlorofluorocarbons (CFCs), the chlorine content in the refrigerant [i.e. fluorotrichloromethane (CFC-11) and difluorodichloromethane (CFC-12)] provided excellent solubility with mineral oil. With the development of the hydrofluorocarbon (HFC) refrigerants to replace CFCs, mineral oil had little to no solubility with the new refrigerants [e.g. 1,1,1,2-tetrafluoroethane (HFC-134a)]. Therefore, new synthetic based lubricants such as polyalkene glycol-based oil (PAG) and polyolester-based oil (PO) were developed. Although the new synthetic oils had better solubility with the HFC refrigerants, the solubility was still not as good as that of CFCs and mineral-based oils. Furthermore the PAG and PO oils are not as effective as lubricants, and additive packages must be mixed with the synthetic oils to improve their lubricating performance. Refrigerant gases other than HFCs, such as non-fluorinated hydrocarbons and carbon dioxide (CO2), have been proposed. Non-fluorinated hydrocarbons have excellent solubility in MOs, however no suitable lubricant has been found for CO2 refrigeration and air-conditioning compressors.
Ionic liquids have been described as potential lubricants. Wang et al [WEAR (2004) 256:44-48] and Ye et al [Chem. Comm. (2001) 21:2244-2245] describe the tribological properties and predicted lubricant functionality of alkylimidazolium hexafluorophosphate and alkylimidazolium tetrafluoroborate, respectively. WO 05/35702 discloses the heat resistance and friction characteristics of lubricating oils containing ionic liquids as base oils.
A need nevertheless remains for lubricants that are well suited for use in conjunction with the current and new refrigerants that are used in a vapor compression system. A lubricant with high solubilty for HFCs and CO2, for example, would be desirable to replace, or be mixed with, traditional synthetic oils such as PAGs and POs to increase the operating lifetime of vapor compression systems. As a result, ionic liquids are disclosed herein, for use with various refrigerants in a vapor compression system, as type of compressor lubricant having a balance of properties that is superior to that of the current synthetic lubricants.